Just sharing my experience upgrading from a Tovp Premiere with a WD MyBook DVR Expander with Tivo Stream to a Roamio Pro with a new WD MyBook in case this helps others as I had gotten some good tips from this forum.
My setup is Comcast and was upgrading replacing a Premiere to a Roamio into the same location in the entertainment center (no multi-room setup and one TV). The Motorola M-Card CableCARD I got a few years ago originally with the Premiere worked great. Just had to call Comcast and the re-pairing worked fine after being on the phone with Comcast for about 15 minutes calling the number which came up on the Tivo - didn't try the pairing-specific 800 phone number on the Roamio instructions. I was going to install the CableCARD later initially per the printed quick start guide, but the Roamio prompted it and worked fine.
The device to device transfer over a Gigabit Ethernet hub worked well overall - transferred about 150 hours of HD in a bit over 24 hours and didn't lose any shows. Also, someone mentioned that the date of the recordings got lost in the transfer, but it didn't happen for me as it preserved them.
I initially pulled the CableCARD, cable coaxial cable and the HDMI cable from the old Premiere onto the new Roamio and left them side by side during the setup/install. Then did the guided setup including the CableCARD re-pairing, and then started the Tivo to Tivo transfer.
I got a network error and the device to device transfer stopped once mysteriously too, but seemed like the Tivo was doing background tasks as the menus changed from SD to HD on some sub-menus. Just forced a connection to the Tivo service a few times and everything worked by trying it twice.
The season pass transfer from the old to the new on the Tivo.com website did't work at all (like a lot of people warned), so ended up doing that manually, but found some new channels while doing it and ended up cleaning up the season passes too - considered it spring cleaning.
Probably the oddest thing was that since the units were next to each other, the universal remote that I programmed with the old Tivo (actually did it for a older TivoHD) kept being recognized by both the Roamio and the Premiere.
I did have to manually delete the To Do list and Season Passes from the old Premiere too, but did it through the Tivo app on an iOS device for ease. Otherwise, the Premiere kept trying to record without a signal.
The Tivo app on my iOS devices worked well switching from the Premiere to the Roamio seamlessly. No glitches here at all.
After the transfer was completely done, then installed the new WD MyBook DVR Expander and that didn't take more than 5 minutes to recognize and install which was smooth and easy.
Then plugged in my Slingbox 500 into the new Roamio and everything worked great there as well with no tweaks needed.
The biggest gripe I had in the entire experience was that I had a friend order the Roamio Pro through tivo.com since he had a great corporate discount, and had to have him call them to transfer the ownership and had a bad CSR person via chat. What a pain it was, but after my friend called the main 800 Tivo number, the tivo.com website had the new Roamio show up extremely quickly onto my account.
Now going to give the old Premiere to a relative. And the family noticed the better quality of the HD Tivo main screen and enjoying the 6 tuners and the extra space for 600+ hours of HD. The box size seemed pretty similar but the front of the Roamio looks cheaper than the Premiere.
Let me know if I can answer any specific questions from anyone.